Upper mantle flow beneath - Pages perso de

1
Guilhem BARRUOL
T23A-1201
I. UPPER MANTLE ANISOTROPY
δt +
φ (azimuth of the fast S wave)
Upper mantle
PS
upper mantle
(V1)
ScS
(V2)
We use SKS and SKKS phases from distances in the
range 85° to 145° to ensure no interference with other
seismic phases. Only events with a signal-to-noise
ratio > 3 were analyzed. We used the method of
Silver and Chan (1991) and also the SpliLab software
Where anisotropy
is observed
Seismic event
Crust
SS
ScS2
S
Lower mantle
S
Anisotropic
SKS
D"
180
(Wuestefeld et al., 2007) to compute the
splitting parameters: the polarization of the
fast shear-wave, φ and the delay time between
the fast and slow shear wave, δt.
1
54˚
90
HURE
event 27/07/03
SKS
HURE
φ = -48°, δt = 1.4 s
60
100˚
fast axi s
0
ya
0
-30
-60
-5000
-20
0
2000
20
-1
40
5
10
15
20
W-E
25
1
BAYN
event 27/07/03
SKS
0
-90
0
1
2
90
3
OKTB
fast axi s
ARSH
KIRN
52˚
0
-60
0
20
-1
40
0
5
10
15
20
25
-90
W-E
1
SKS
0
1
2
3
4sec
90
DALA
event 27/07/03
DALA
φ = -31°, δt = 1.2 s
60
0
110°E
115°E
60°
100°
120°
Sa
meters
Bolnay
tay
Han
gay
Bogd
GobiAltay
4
ika
0
Ba
Hantay
2000
-90
0
45
90
135
180
225
Backazimuth
270
ULN, best 2 layers model:
φupp = 72° δtupp = 1.0 s
φlow = 136° δtlow = 0.8 s
Mongolia
1000
330
0
30
60
300
China
A
90
3s 2s 1s
delay time
45
135
46˚
BUGA
TSET
ULN
BUMB
UULA
DALA
ALTA
44˚
180
150
a
225
Backazimuth
best φupp
Gobi Alta
i
Individual splitting measurements at the MOBAL station
together with the TLY and ULN IRIS permanent broad band
station. Fair (grey) and good (black) quality are plottted.
The splitting are characterized by their azimuths and the
delay time (length of the segment).
200
270
315
360
d
250
e
200
150
ULN,
best
150
1000 models
100
100
0
0
50
20
40
60
80
100 120 140 160 180
Upper layer fast direction
0
0
20
40
60
80
We performed direct modelling of 2
anisotropic layers that we compared with our
observations.
a: anisotropic parameters of the 50 good
splitting measurements in a polar diagram.
best φlow
120
210
180
Delay times
2
A
H
S
2.0
48
46
YN
A
B
DR
BA
44
B
UM
B
TA
L
A
TB
OK
0. 5
52
50
48
46
Latitude
44
DALY
DLY2
SHA2
Splitting parameters obtained at ULN
(Ulaanbaatar, Mongolia, more than 10 years
of data) display backazimutal variations that
suggests the presence of 2 layers of
anisotropy.
90
50
(
SKS, lithospheric
thickness and
volcanism
Tomographies (Mordvinova et al., 2007) and
petro-geochemical analyses of mantle xenolith
(Ionov, 2002) suggest that the Siberian lithosphere is
around 200 km thick whereas it is less than 100 km
thick beneath central Mongolia.
The lithospheric structures clearly wrapped around
the Siberian platform that acted as a rigid nucleus
during the Paleozoic block accretion. The NW-SE φ
trend observed across the Hangay dome could then
be partly related to old lithospheric deformation.
Interestingly, the largest δt are observed above the
low velocity anomalies in the upper mantle
tomography, compatible with a strong asthenospheric
participation in the signal. The anisotropy beneath
the Hangay dome could therefore result from two
lithospheric and asthenospheric anisotropies of
similar orientations, their individual effects adding
together.
OVGO
c
0
0
360
52
)
p
up
TLY
TUSG
ULN
50
240
0
315
U
1.5
BAYN
Han
gay
dom
e
1
b
LN
(l
SHAR
2
-60
TLY
ULN
30
O
TA
L
A
)
ow
BADR
4sec
3
-30
Russia
3
108˚
Two layers of anisotropy beneath ULN
TH
MU
ZI
Al
45°N
n
Hövsgöl
50°N
3000
l
ya
2
The two stations located in the Gobi-Altay range (ALTA
and DALA) are characterized by intermediate δt (1.3 s)
and statistically different φ directions (N030°W) than the
Hangay stations.
60
140°
1
Delay times
20° 80°
0
Stations on the Hangay dome are characterized by
homogeneously NW-SE trending φ and high δt (2.8 s at
BAYN, 1.9 s at BADR, 2.3 s at BUMB), suggesting a
strong and coherent mantle deformation beneath the
stations.
fast Azimuth
55°N
-90
W-E
30
90
Siberian
Craton
0°
20
TLY exhibits a complex anisotropy pattern which is not
compatible with a single anisotropic layers but suggests
strong mantle heterogeneities beneath this area.
40°
20° INDIA
10
Except at the northernmost stations where only few data
were available, clear anisotropies have been detected in
central Mongolia.
4000
EURASIA
0
Number of models
60°N
105°E
-1
40
48˚
From April to October 2003, 18 three-component, broadband stations
from the French Lithoscope program were deployed along a NS
trending profile extending from the southern Siberian platform to the
Gobi-Altay range, crossing the southwestern tip of the Baikal rift and
the whole Hangay dome. This temporary seismic deployment took
place in a larger project combining other observations such as geodesy
and seismotectonics in order to constrain the crustal and mantle
structures but also the past and present-day tectonic processes
occurring in the Baikal-Mongolia system.
100°E
20
Examples of splitting measurements obtained for event 2003/208
(27-Jul-2003, 02:04, Mw=6.6, lat. -21.08°N, long. -176.59°E, depth
212km, backazimuth 110°) at stations HURE, BAYN and DALA.
Mongolian-Baikal Lithosphere
seismological Transect
95°E
50˚
-60
0
HURE
0
-30
-4000
-20
90°E
fast axi s
0
S-N
II. THE MOBAL NETWORK
TORI
-30
-3000
-20
106˚
KAIT
4sec
30
0
B
T
K
1.0
n
BAYN
φ = -58°, δt = 2.9 s
60
0
104˚
Siberian Platform
Sa
30
0
102˚
2.5
δt (s)
Map of Eigenvalues
Particle motion before (- -) & after (-)
S-N
Shear wave splitting analysis is a way to scan the active or frozen upper
mantle flow. It is directly induced by olivine preferred orientation and its
intrinsic anisotropy. In oceanic domains, upper mantle anisotropy is likely
controlled by the asthenospheric flow gradually freezing at the bottom of the
lithosphere. In continental domain, the deformation may be partly frozen within
the lithosphere and related to ancient tectonic processes but also present in the
underlying asthenosphere and related to present-day processes, such as the
plate motion.
5000
Corrected Fast (-) & Slow(- -)
98˚
al
Outer core
UL
80
3.0
Original radial (- -) & transverse (-) components
N
60
30
Mongolia represents the northernmost area affected by
the Indian-Asia collision and is actively deformed along
transpressive belts closely associated to strike-slip faults. In
order to investigate the deep mantle deformation beneath
central Mongolia and its relation with the surrounding major
structures such as the Siberian craton, the Altay range and
the Baikal rift, a NS trending profile of broadband seismic
stations has been deployed in summer 2003 from the
southern Siberian craton to the Gobi-Altay range, crossing
the whole Hangay dome. Mantle flow is deduced from the
splitting of teleseismic shear waves such as SKS phases. In
eastern Mongolia, the permanent station ULN in
Ulaanbaatar reveals the presence of two anisotropic layers,
the upper one being oriented NE-SW, close to the trend of
the lithospheric structures and the lower one NW-SE, close
to the trend of the plate motion. Along the NS profile in
central Mongolia, seismic anisotropy deduced from SKS
splitting reveals a homogeneous NW-SE trending structure,
fully consistent with the observations performed in the
Altay-Sayan in western Mongolia. Since the observed delay
times of 1.5 to more than 2.0 s suggest coherent mantle flow
over large mantle thicknesses and since the observed fast
directions are parallel to the trend of the lithospheric
structures but also close to the trend of the plate motion, we
propose that both the lithosphere and the asthenosphere may
add their anisotropic effects beneath central Mongolia. In
order to interpret the slight clockwise rotation of the fast
directions relative to the plate motion vector, we propose
that the root of the Siberian craton could deflect the
asthenospheric flow around its southwestern side. GPS
vectors and SKS splitting depicts a similar trend beneath
central Mongolia, suggesting that the block “escaping”
toward the east moves consistently with the lithospheric and
asthenospheric mantle flow. A strikingly different behaviour
is observed in western Mongolia: The GPS vectors trend NS
whereas the fast SKS directions trend EW, suggesting that a
decoupling occurs somewhere between the upper crust
moving northwards and the mantle flowing eastwards.
120
V. CONCLUSION
SOUTH
Fast split directions
ik
Isotropic
NORTH
140
100
Polarized incident
SKS wave
4000
ABSTRACT
160
φ (°N)
SKS wave splitting
IV. DISCUSSION
III. SHEAR WAVE SPLITTING MEASUREMENTS
S-N
(1) Géosciences Montpellier, CNRS,
Université Montpellier II, France.
barruol@gm.univ-montp2.fr
bokelmann@gm.univ-montp2.fr
(2) Géosciences Azur, UNSA/CNRS
Valbonne, France.
deschamps@geoazur.unice.fr
(3) Université de Bretagne Occidentale,
CNRS, Plouzané, France.
jacdev@univ-brest.fr
jperrot@univ-brest.fr
(4) Institute of the Earth's Crust Irkutsk,
Russia.
mordv@crust.irk.ru
(5) Research Centre of Astronomy &
Geophysics, Ulaanbaatar, Mongolia.
ulzibat@rcag.url.mn
dugarmaa@rcag.url.mn
Upper mantle flow beneath
the Hangay dome, central Mongolia
3
Julie PERROT ,
4
Alexandre ARTEMIEV ,
5
Tundev DUGARMAA
1
Götz BOKELMANN
Ba
2
Anne DESCHAMPS ,
3
Jacques DEVERCHERES ,
4
Valentina MORDVINOVA ,
5
Munkhuu ULZIIBAT ,
100 120 140 160 180
Lower layer fast direction
b and c: apparent variations of φ and δt as a
function of the backazimuth of the incoming
wave, for the best two-layer model found and
for our observations.
d and e: histograms showing the distribution
of the upper and lower fast directions of the
1000 best two-layers models amongst more
than 1 million models tested.
GPS and SKS : crust mantle (de)coupling
This map of Mongolia and surrounding
areas presents the mean SKS splitting
measurements available in the region,
together with the GPS vectors from Calais et
al. (2003) and the absolute plate motion
vectors calculated from HS3-Nuvel1A.
In central Mongolia, the SKS fast
directions and the GPS vectors are rather
close from each other, and also to the
present-day APM, suggesting that the
crustal block escape is coherently
accompanied by a present-day mantle flow.
In western Mongolia, GPS vectors are
close from NS and SKS φ close from EW,
suggesting a complete decoupling between
the crustal tectonics and the underlying
mantle deformation.
Siberian Platform
APM
80 Km
Ha
B
ng
ai
l
ka
ay
ULN
a
a: At lithospheric depth (e.g., 80 km), the
main lithospheric structures wrapped around
the Siberian craton. The anisotropic upper
layer at ULN trends NE-SW and NW-SE
across the hangay dome.
86˚
88˚
90˚
92˚
94˚
96˚
98˚
100˚
102˚
104˚
106˚
108˚
110˚
112˚
54˚
52˚
50˚
48˚
Acknowledgements
This work was performed with funding from the French CNRS (Centre
National de la Recherche Scientifique) PICS (International program for
scientific collaboration) program 1251, and DYETI CNRS contract and
Lithoscope French mobile instrumentation. We also thank the Institute of
the Earth's Crust RSA, Irkutsk, RCAG, Ulan Bator and DASE/CEA, France
for their support. The data will be available in 2008 at the site
http://bdsis.obs.ujf-grenoble.fr/ maintained by C. Pequegnat.
46˚
44˚
42˚
The MOBAL temporary deployment across central
Mongolia allowed us to characterize upper mantle
anisotropy beneath most stations. The anisotropy pattern
is clearly defined across the Hangay dome, with NW-SE
trending φ and relatively large δt. Geophysical and
geochemical arguments favors a lithosphere thinner than
90 km beneath central Mongolia, contrasting with a thick
(at least 200 km) Siberian platform. This suggests that the
splitting across the Hangay dome cannot be explained by
a lithospheric deformation alone. We propose that the
observed anisotropy reflects a combination of both
lithospheric and asthenospheric deformation, the first
resulting from the long-lasting geological evolution and
recent (Cenozoic to present-day) deformation of the
Mongolian lithosphere along active margins and along the
present-day large-scale strike-slip faults and the second,
asthenospheric, being related to the present-day Eurasian
plate motion.
Our preferred model suggests that the NW-SE trending
φ observed on the Siberian platform could be dominated
by an asthenospheric flow and that both the lithospheric
and asthenospheric flow have similar orientations beneath
central and western Mongolia and may add their effects to
explain the strong delay times observed. On the other
hand, beneath eastern Mongolia, the lithospheric
structures clearly rotate to a NE-SW dominant direction
inducing a two-layered structure that is well imaged
beneath Ulaanbaatar.
The very homogeneous anisotropy pattern characterizing
the whole central and western Mongolia region contrasts
with the different behavior of these two regions as seen by
the GPS velocity field, suggesting a complete
crust-mantle decoupling beneath western Mongolia
whereas a better coupling seems to exists in central
Mongolia.
References
Absolute plate velocity
(HS3-Nuvel1A)
GPS velocity
(Calais etal., 2003)
Shear wave splitting
1.0 s
2.0 s
5 mm/yr
10 mm/yr
(Dricker et al., 2002)
(Gao et al., 1994, 1997; Vinnik et al., 1992; Silver et al., 1991)
Schematic presentation of the upper
mantle flow that could explain the seismic
anisotropy observed in Mongolia. In this
model are summarized the SKS splitting,
the absolute plate motion (APM) and the
GPS vectors.
Such model explains the NW-SE
trending anisotropy and the large δt
across the Hangay dome and the two
layers of anisotropy beneath Ulaanbaatar.
150 Km
Calais, E., Vergnolle, M., San'kov, V., Lukhnev, A., Miroshnitchentko, A., Amarjargal, S.
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ULN
b
b: At asthenospheric depth (e.g., 150 km
for central Mongolia), the active mantle
flow is likely deflected by the thick Siberian
cratonic root. Such lower anisotropic layer
trends homogeneously NW-SE.
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